The production of relief images for flexographic printing is generally carried out by imagewise exposing the photosensitive layer of a flexographic printing precursor using suitable imaging radiation such as UV radiation. Unexposed areas of the photosensitive layer are washed off (developed or “washed out”) using a suitable developer or processing solution while exposed, and crosslinked areas are left intact. Residual developer is generally removed by evaporation and if necessary, the developed surface is treated to remove tackiness. A resulting flexographic printing plate having a relief image can be wrapped around a cylinder on a printing press and used to transfer ink to a suitable substrate composed of various papers, polymeric films, fabrics, ceramics, and other materials. Alternatively, the resulting flexographic printing member can be a flexographic printing sleeve that is slid onto a suitable mandrel and used to similarly transfer ink to a substrate.
While the non-exposed photopolymer may be soluble in a variety of organic solutions, only some of those organic-based developers do not damage or swell the crosslinked portions while cleanly removing the non-crosslinked portions. Swelling will eventually cause the relief image to deteriorate in the processing bath and the processing solution will have to be changed frequently due to the build-up of sludge. A wide variety of organic solvents and mixtures of solvents have been provided in the art and industry for this purpose.
However, for a variety of environmental and safety concerns, there has been a strong incentive in the industry to avoid the use of solvent-based developers and to find aqueous-based flexographic developers that will provide the same quality of processing as organic solvent-based developers while avoiding environmental concerns associated therewith. This has been difficult to achieve because of the nature of various photopolymer compositions designed for flexographic printing precursors. Not just any type of aqueous solution, with or without water-miscible organic solvents will meet all of the rigorous processing and performance requirements.
For example, it is necessary that effective processing of an imagewise exposed flexographic printing precursor remove all of the non-exposed photopolymer, leaving relief images with well-defined boundaries (high resolution) and appropriate relief depth. Not just any developer, aqueous or non-aqueous, can be effective with a given photopolymer composition.
Moreover, as an aqueous flexographic developer is used continuously to process numerous imagewise exposed precursors, the pH of the developer can change thereby causing more photopolymer components dispersed therein to come out of solution and to re-deposit on the surface of the relief image. This reduces relief image resolution and results in serious image defects during printing. Such dispersed photopolymer components also can stick to brushes used during processing and cause “scum” on the relief image member. This problem increases as the developer pH becomes more acidic.
U.S. Patent Application Publications 2007/0117039 (Wada et al) and 2012/0288682 (Inoue et al.) describe aqueous developers used for solubilizing non-exposed photopolymer compositions, which aqueous developers include one or more nonionic or anionic surfactants and a suitable pH controlling agent.
U.S. Pat. No. 9,005,884 (Yawata et al.) proposes to solve such problems with an aqueous flexographic developer that comprises both saturated and unsaturated fatty acids at a 20:80 to 80:20 weight ratio, along with an alkali agent. Depending upon the particular ratio of saturated and unsaturated fatty acids, such compositions (also known as “soaps”) may not sufficiently “develop” the exposed flexographic printing precursors, thereby leaving debris on the resulting flexographic printing plate, leading to severe printing defects in the resulting impressions.
In addition, it has been found that as the pH of such aqueous developers becomes more acidic with continuous use, fatty acids incorporated therein are converted from their basic (ionic) form to their acidic form and their solubility in water and ability to form micelles are seriously reduced. When this happens, the debris from the non-exposed photopolymer is less solubilized in the absence of appropriate micelles. Both precipitated (solid) fatty acids and photopolymer debris float throughout the aqueous flexographic developer, collect on brushes used during processing, and end up as scum on the relief image of the printing plate. These are unacceptable results in the industry and it to address these problems that the present invention is directed.